This invention relates to a process for producing a coating on aluminum, and more particularly, for producing a solar selective coating.
Aluminum has been anodized in the past both to protect the substrate against corrosion as well as to provide an aesthetically pleasing surface suitable for indoor or outdoor uses without further covering or coating. Most notable of such applications is the use of anodized aluminum, both natural and colored, on the outer curtain walls of buildings. For such applications, it has become conventional to provide thick anodized coatings, i.e. as much as 15 microns or more, on the aluminum surface. The porous coating is then sealed using processes well known to those skilled in the art.
In more recent years, the search for a broader and more pleasing range of colors has lead to the insertion of an AC coloring step between the anodizing and sealing steps using various metal salts, typically metal sulfates. Such a process is described in Asada U.S. Pat. No. 3,382,160. An improvement on this process, as described in Asada U.S. Pat. No. 4,022,671, inserts an electrolytic processing step in phosphoric acid subsequent to the sulfuric acid anodizing and prior to the electrocoloring.
While such processes have yielded interesting products for architectural uses, coatings produced by such processes have not been found useful in the production of solar selective coatings. Because of the emphasis on energy conservation and energy sources, considerable interest has been shown in the production of solar coatings. Apparently, the lack of solar selectivity, i.e. high absorption and low emittance, is due to the thickness of the coating produced by these processes.
French Demande No. 2,360,051, entitled "Solar Heat Collector Containing a Coating Selectivity Absorbing Solar Heat Energy", by Showa Aluminium K.K., describes a process for producing a solar selective coating on aluminum using phosphoric acid anodizing followed by AC electrocoloring. While materials produced by this process are reported to exhibit excellent optical properties, the anodizing times of 15 to 20 minutes reported using 15 to 20% H.sub.3 PO.sub.4 make the process economically impractical for continuous anodizing of coil where it is desirable to have much shorter residence time in the anodizing bath. The AC electrocoloring step is also reported as a 15 to 20 minute step, depending upon the particular metal salts used. The process is described as being carried out under constant current conditions of 1 to 1.5 amperes per square decimeter.